NASA Facts

National Aeronautics and

Marshall Space Flight Center

Huntsville, Alabama 35812

FS-1997-02-002-MSFC May 1997

X-ray Calibration Facility

Marshall’s X-ray Calibration Facility

The X-ray Calibration Facility at NASA’s Marshall Space

Flight Center in Huntsville, Ala., is the world’s largest, most AXAF’s Missionadvanced laboratory for simulating x-ray emissions from AXAF, a Marshall Center-managed orbiting observatorydistant celestial objects. This facility produces a space-like scheduled for launch in late 1998, will begin a mission ofenvironment in which components related to x-ray telescope discovery planned to last at least five years. It joins NASA’simaging are tested and the quality of their performance in two other 10,000-pound-plus orbiting observatories—thespace is predicted. Hubble Space Telescope and the Compton Gamma-Ray Observatory—examining the wonders of the universe.The calibration facility, which includes four buildings Newcomer AXAF will scrutinize objects in the invisiblelocated over approximately a one-third square mile area, is energy range of x-ray radiation. It will study the x-ray­presently being used to evaluate optical elements of the producing birth and death of stars and galaxies, as well asAdvanced X-ray Astrophysics Facility (AXAF). investigate some of the most exotic objects in the universe— spinning neutron stars, mighty quasars and mysterious blackThe evaluation process will verify that the telescope’s holes.mirrors are operating properly and will use familiar x-raysources to measure the performance of the mirrors under AXAF’s sister observatory, the Hubble Space Telescope,controlled conditions. Measurements obtained during the primarily focuses on visible light range events. Comptonevaluation process will allow scientists to correctly interpret examines the invisible portion of the energy range dominatedinformation received from the orbiting x-ray telescope. by gamma-ray-producing phenomena. Pub. 5-560-2 Field of View ±.5 Degrees

Doubly Reflected Four Nested Hyperboloids X-rays Focal Surface

Doubly Reflected X-rays

rs ete M 10

X-rays

Four Nested Paraboloids

Mirror Elements are 0.8 m long and from 0.6 m to 1.2 m diameter.

When findings compiled from the three observatories are AXAF, with 100-times greater sensitivity than the High-considered together, researchers anticipate being able to Energy Astronomy Observatory-2, has the power to detectanswer fundamental questions about the age and size of the far more x-ray sources than its predecessor.universe, the properties of matter under extreme conditionsand the fundamental laws of physics. A small-scale, 16-inch prototype of AXAF’s mirrors was tested in the original facility in 1989. The demonstrationHistory proved that mirrors could be polished to a finish sufficiently smooth to achieve such image precision. In early 1997,In 1975–1976, the original X-ray Calibration Facility at NASA will verify that the AXAF mirrors, which are theMarshall was built for testing the Center’s High Energy largest grazing incidence mirrors ever made, have beenAstronomy Observatory-2 (HEAO-2), also referred to as the fabricated just as precisely as the earlier prototype, and theEinstein Observatory. At that time, the new x-ray imaging facility will be used to calibrate the telescope for spacetelescope was the most sophisticated ever constructed. Its flight. The largest of the AXAF mirrors is approximatelyfindings increased the number of known x-ray sources from 47.2 inches in diameter.hundreds to almost 10,000. Soon after completing the 1989 preliminary testing, Nested Array of construction began on improvements to raise the calibration Hyperboloids Incoming X-ray facility to its present standing as a world-class test site. The Energy previous 1,000-foot guide tube, through which x-ray beams are directed from the generation source to the mirrors where the x-rays are focused, was lengthened by 700 feet, with larger diameter tubing. Other facility upgrades included a new building and control room, a much larger instrument Nested Array of chamber and additional x-ray generators. Paraboloids Focus at The Calibration Facility Complex Detectors The complex is composed of four buildings: The x-ray source building where the x-rays are produced; the x-ray detector building in which the intensity and properties of the x-ray beam are monitored; the instrument vacuum chamber building which houses the instrument vacuum chamber, the control room and clean room; and a mid-range building where gas pumping functions are controlled.

Grazing incidence mirrors—Top illustration shows In the source building, each of three generators createsincoming x-ray energy as it is bent by the mirrors x-rays with distinctly different intensities and wavelengths—toward the focal point; bottom sketch is of nested like those occurring naturally in space—and directs a beammirror pairs. into the guide tube. Instrument X-ray Source 1700-ft Guide Tube Chamber

Instrument

Chamber

Clean Room

Overhead diagram of X-ray Calibration Facility.

As the emissions race through the tube, x-ray scattering is designed to block out diverging rays, so the beams reachingblocked by partitions, or baffles, along the way to the the instrument chamber are virtually parallel.vacuum chamber. X-rays reaching the mirrors inside the testchamber are as nearly parallel as possible, to simulate The stainless steel guide tube is of three diameters—36, 48radiation coming from distant cosmic sources. The rays and 60 inches. Attached to the x-ray source building isgraze off the mirrors and onto detectors mounted at the focal 1,000 feet of 36-inch-diameter tubing that was part of thepoint. To determine the calibration of the mirror pairs and original X-ray Calibration Facility. When the X-raytheir focus, the x-ray images produced are analyzed by test Calibration Facility was upgraded, this part of the guide tubeengineers and scientists. was moved. Four hundred feet of 48-inch diameter and 300 feet of 60-inch diameter tube was added.X-ray Generators The 1,700 foot guide tube is not level along the Earth’sTest facility x-rays of varying intensities and properties are surface. Since the tube is so long, it would actually curveproduced by three generator assemblies in the x-ray source slightly with the Earth’s surface if it had been built “level” atbuilding at the opposite end of the 1,700-foot guide tube all points. So, while it is straight, a level would indicate thatfrom the telescope mirrors in the main building. To simulate the tube is slightly “high” at each end.rays coming from distant stars, different methods of creatingx-rays are used. One method bombards metals such as The inside of the airtight guide tube must be a vacuum. Ifcopper, tin or iron with a stream of high-speed electrons. The even a slight amount of air were present, it would absorb thetest x-rays’ exact wavelength is determined by the energy of test x-rays and prevent them from reaching their mirrors.the electrons and the atomic properties of the specific metalserving as the target. The chamber and guide tube can be isolated by solid gate valves. This allows the guide tube to be used independentlyDiagnostic equipment gives precise readouts on the x-rays’ of the instrument chamber.wavelengths and intensities. Instrument Vacuum ChamberGuide Tube The stainless steel, instrument vacuum chamber—the heartMost celestial objects AXAF will view are millions—even of Marshall’s X-ray Calibration Facility—is located in thebillions—of light years from Earth. With these originating instrument chamber building. It is thermally controlled,sites so far away and no atmosphere in space to diffuse the capable of simulating widely diverse temperatures within ax-ray stream, the emissions appear as a single-point source— vacuum. It can be heated to evaporate or “bake-out”with the beams absolutely parallel. contaminated materials or cooled with liquid nitrogen. Researchers can evaluate how hardware will operate underEven though Marshall’s generators concentrate the x-rays to extreme temperature conditions, ranging from –40 tostream from nearly a single point, after leaving that source 160 degrees Fahrenheit, in the vacuum environment of space.the emissions radiate in all directions. The long guide tube is

FWD CONT HETC Storage Position

X-rays Cold Shroud

Instrument chamber with test mirrors and detectors inside.

The chamber’s interior test volume measures 60 by 20 feet After this process, the final vacuum is two ten-billionths of(18 by 6 meters)—large enough to hold anything that will fit the pressure outside—very similar to the vacuum in space.into the Space Shuttle’s cargo bay. For the AXAF optics tests, the chamber’s temperature willOpening off the chamber is the “clean room,” where air is be about 50 degrees Fahrenheit—slightly below the internalconstantly cycled through filters to remove dust and temperature that will be maintained in the telescope as itimpurities that might contaminate the telescope instruments. orbits the Earth.It is a Class 2000 clean room. No more than 2,000 dustparticles, 0.5 micron or smaller, are contained in a cubic foot Once a hard vacuum is achieved, it can be maintainedof air—a five-fold improvement since the facility was indefinitely by test crews working 24 hours a day.upgraded. Test ControlTo protect the carefully aligned optics from vibrations, theinstrument vacuum chamber is mounted directly to the Engineers control and monitor test activities from a controlbuilding’s floor. The optical supports inside the chamber are room with a three-part, state-of-the-art graphic panel.supported independently by 22 stainless steel pillars. Thepillars pass through the chamber’s wall. Between the Alarms at critical locations in the facility are available tobuilding floor and a separate, 5-foot-thick concrete alert controllers to problems. Near the control panel,foundation that rests on 2 feet of compact sand, is an information-gathering electronic and computer equipment isisolating air gap. The support pillars, air gap, concrete set up for scientists evaluating the mirrors and detectors.foundation and compact sand form a seismic isolation pad. During a typical test series, x-rays are generated for specifiedSeparating the pillars and chamber are 22 bellows which periods. Then these trials are suspended while scientistsmaintain the vacuum seal and reduce the transmission of any analyze their findings, but the vacuum in the chamber isvibration from the chamber to the optics inside. usually maintained. Testing can be resumed if scientists consider further evaluation necessary.The evaluation of AXAF’s four mirrors and scienceinstruments will begin in December 1996 and be completedin May 1997. The mirrors and detectors used to test themwill be assembled in the clean room on mounting platforms,called test benches, and then rolled inside the chamber. Next, The X-ray Calibration Facility construction wasthe mirrors will be aligned with the x-ray source at the end of accomplished by Universal Construction Company ofthe guide tube—one-third of a mile away. Huntsville.

Vacuum Pumps Tests are being conducted by the X-ray Calibration Systems Division of Marshall’s Systems Analysis and IntegrationUsing a series of vacuum pumps, the instrument chamber, Laboratory, supported by the contractor teams and otherguide tube and x-ray generator are pumped almost totally laboratories of the Marshall Science and Engineeringfree of air molecules. While more traditional mechanical Directorate.pumps do most of the work, special high-vacuum pumpsfinish the job. The surfaces of these pumps are chilled to After AXAF system testing concludes, the X-ray Calibration–440 degrees Fahrenheit. They literally attract and trap the Facility will continue to serve as a valuable national resourceremaining molecules of air. for evaluating x-ray telescope mirrors, International Space Station elements and other NASA flight hardware.Achieving a hard vacuum in the chamber takes about fourhours. However, to bring the temperature to the level andstability desired requires 20 to 24 hours.